[The transmission of tuberculosis in a cram school]

The specificity of the tuberculin skin test (TST) is influenced by the previous BCG vaccination during childhood in Japan. The QuantiFERON (QFT) is expected to detect latent infection among tuberculosis contacts, however, its use is still limited. We report a case of TB outbreak that caused by a private lesson teacher. His sputum was found to be acidfast bacillus-positive (Gaffky 4) by smears examination. We evaluated the risk factors of TB infection using TST among contacts.

A total of 184 persons were examined by a TST. These results were used to estimate the rate of TB infection by a mixture model. Logit analysis was also performed to estimate the number of infected person by using the relation between the positive rate of QFT and the size of erythema using a past report. We applied the Wells-Riley mathematical model of airborne transmission to calculate the risk of infection.

As compared with persons who had less contact with the index TB case, the estimated rate of TB infection was higher among persons who had close contact during the month before the index case started medical treatment (5% vs 23%). According to the model, the case added 1.1-4.0 quanta per hour (qph) to the room during a month before starting the treatment.

These results suggest the importance of an early diagnosis of the index case to prevent TB infection.